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In the presence of oxygen (left), a nickel (green) atom from the CODH metallocluster sits apart from its iron (orange) and sulfur (yellow) partners. When the enzyme is reduced, the atom moves back into the conventional cube-like structure (right).

As the window to limit the consequences of climate change appears to be narrowing, scientists are considering a range of technologies to slow warming, including those that pull carbon dioxide from industrial emissions or from the atmosphere. Some scientists would like to enlist the help of an enzyme that can convert CO2 into carbon monoxide, which could be used as a feedstock for industrial reactions. But most of these enzymes are too delicate for industrial use: They fall apart in the presence of oxygen.

Credit: eLife

This CO2-converting metallocluster changes between its conventional cube-like structure and an oxidized state. The authors describe the movement of nickel (green), iron (orange) and sulfur (yellow) atoms as a "molecular cartwheel."

Now, researchers from the U.S. and France report a structure-shifting mechanism that protects one of those CO2-converting enzymes from oxidative inactivation. The study, led by Catherine Drennan of Massachusetts Institute of Technology, also reveals one of the most dramatic re-arrangements observed in metalloclusters, which were generally thought to be static cofactors in enzymes (eLife 2018, DOI: 10.7554/eLife.39451).

The CO dehydrogenase (CODH) of interest comes from a common bacterium called Desulfovibrio vulgaris and has been known to tolerate oxygen better than its relatives. When analyzing the enzyme’s CO2-converting metallocluster consisting of nickel, iron and sulfur, Drennan and coworkers found two conformations in two different protein batches. One was a textbook cube-like structure, and in the other its nickel atom had switched positions with one of the iron atoms.

“Someone might have thought, ‘Oh, this must be a mistake,’ ” says Stephen Ragsdale of University of Michigan Medical School, who studies CODHs but was not involved in this study. But these researchers did a cool experiment, he says. They reduced the Ni-swapped protein, returning the metallocluster back to its conventional form.

Ragsdale would like to see follow-up experiments that examine the timescale of this structure restoration as well as activity recovery. A better understanding of this mechanism will help design inexpensive metal systems that mimic key properties of CODH without the oxygen sensitivity, he says. For example, an ideal system could catalyze CO2 reduction under standard temperatures, pressures, and atmospheric concentrations of the gas without requiring extra energy input.

Drennan is working to decipher the metallocluster’s assembly process and to uncover other ways that the bacterial CODH stays stable around oxygen. Although her group focuses on basic science, she observes, “As people become more and more concerned about climate change and other things, there’s a renewed interest” in these enzymes and their clusters because they represent alternative and energy-efficient ways to fix carbon.